HBASE-7970 Improve file descriptor usage: currently, there are two file descriptors per storefile

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1476677 13f79535-47bb-0310-9956-ffa450edef68
Merge remote branch 'origin/trunk' into trunk-gd
Merge remote branch 'origin/trunk-gd' into fb-YAHOO-317-trunk-gd